In realizing a low on-resistance of a device in operation and a high speed switching, a high electron mobility transistor (HEMT) using a group III-V compound semiconductor in which a two-dimensional electron gas (2DEG) may be formed in a channel layer. HEMT having 2DEG in the channel layer under a gate electrode may have a normally-on characteristic such that a current flows between a source and a drain without being applied with a voltage to the gate. However, improvements in such configuration of HEMT remain desired.